1. Field of the Invention
The present invention relates generally to a light-emitting diode (LED) circuit, more specifically to a circuit for driving LED with variable output current.
2. Description of the Related Art
Light-emitting diodes (LEDs) are used in various display devices including LED video billboards. Display devices such as LED video billboards may include a large number of LEDs to produce high resolution images or videos. Brightness of the LEDs in such display devices fluctuate in response to current in the LEDs. Especially in large LED display devices, minor changes in the current may result in flickering visible to human eyes. Therefore, the current in the LED must be regulated by a LED driver circuit to maintain the current constant in the LED.
The LEDs in the LED video billboards may operate under different current conditions. Due to such different current conditions, the turn-on time or the turn-off time of each LED in the LED video billboards may also differ. Such differences in the timing may result in blurred images that degrade the quality of the images or videos displayed on the LED video billboards. Therefore, in order to provide dynamic video images of high quality, a mechanism is needed to synchronize the switching operations of the LEDs across different LEDs in the LED video billboards.
LED driver circuits may be used to control one or more LEDs. The LED driver functions as a current source or a current sink that regulates a constant current to flow in the LEDs despite changes in voltage conditions or variations in other operating conditions. Typically, the LED driver circuits consist of digital components that communicate with other digital circuitry in a display device and analog components for controlling the current in the LEDs. The LED driver circuits may be designed to control more than one LED according to digital signals received from other digital circuitry in the display device.
Most LED driver circuits for driving LEDs are based on high voltage CMOS (Complementary Metal-Oxide-Semiconductor) or bipolar junction transistors (BJT). The LED driver circuits based on high voltage CMOS or BJT are less sensitive to variations in the current and voltage, and therefore, are relatively easier to design. Most of other components associated with the LEDs, however, operate in low voltage (e.g., 5V). Therefore, a separate power source may be provided to the LED driver or the low voltage must be pulled up to a higher voltage to operate the LED driver.